Generally DC power supplies such as fuel cells or batteries only have a low voltage difference per cell. This means the batteries or cells are often connected in series in order to obtain a useful working voltage. It is customary to provide some form of monitoring circuit for such a chain or ladder of cells.
A fuel cell is a device, which converts chemical energy into electrical energy, where the functionality is as follows: A fuel on the anode side and an oxidant on the cathode side react in the presence of an electrolyte generating a charge difference between the anode side and the cathode side. An appliance can now be connected via wires or cables to the anode side and the cathode side. A fuel cell system comprises several, at least two fuel cells, wherein each of the fuel cells is connected to the adjacent fuel cell in a serial connection. In consideration of the voltage needed an appliance can be connected to a number of fuel cells connected in series to provide the appropriate voltage. In case of a discontinuity in the electricity supply the applied voltage to the appliance is lowered and therefore the function of the appliance is affected. Even worse, the fuel may be supplied to a fuel cell even though it is not functioning correctly. As the fuel is not consumed there is a danger it may be exhausted to the environment.
One of the solutions of this problem is a cell voltage monitor (CVM), which is commercially available. Each of the fuel cells needs to be connected to the CVM separately.
The CVM detects a deviation of the voltage of a fuel cell from an average voltage of a fuel cell system and takes actions like notification of the actual voltage or a shutdown of the fuel cell system. During start-up and shutdown the voltage of a fuel cell can drop under the value of 0 V, which means the CVM must be able to distinguish between transient states of the fuel cells, a failure of the CVM or of a fuel cell. Hence a connectivity check of the wires or cables of the CVM is required.
Conventional solutions of the prior art are for instance demonstrated in JP 2006/153758, which proposes a fuel cell system comprising four fuel cell groups with two fuel cells in each fuel cell group, which are connected in a serial connection comprising a wire connected to each fuel cell. Each of the wires is directed to a low-pass filter, which is grounded and supplied with electricity by a separate voltage supply. Since each of the low-pass filters are provided with a separate voltage supply, two switches are needed to charge and discharge the capacitance of the low-pass filters, which makes a connectivity check complex and expensive.
US 2007/0196707 A1 proposes a fuel cell system, which is divided into several fuel cell portions. A connection check is made by measuring the voltage of each fuel cell portion compared to a ground, and amplifying the measured voltage via two band-pass filters to a signal processing unit. The voltage over the fuel cell portion is then compared with a mean voltage of the fuel cell system, wherein a deviation is an indication of a discontinuity of the electricity supply. A disadvantage of this embodiment is that only the voltage of a fuel cell portion compared to the ground can be measured and not the voltage of a single fuel cell, which makes it difficult to localize the discontinuity in one of the wires.